China signalled its intentions regarding launch date well in advance. Here is a description of how engineers controlled the Tiangong 1 orbit to ensure it was ready for when Shenzhou 10 was ready to go into space.

A detailed summary of all Tiangong 1's orbital manouevres can be found here.

Evolution of the Launch Date

The chart below originally pointed to a possible mission during 2012 December. It is a record of how Tiangong 1 is heading back to the Shenzhou operating height from its storage orbit. For any particular date, there is a forecast that is based on the initial decay rate being at the current level. The projected date is compensated to allow for decay increasing as Tiangong 1 gets lower and experiences greater atmospheric drag.

In 2011 November, when Tiangong 1's orbit was pushed higher for the wait after the Shenzhou 8 mission, it went to an orbit averaging 374 kilometres from where it was steered by a combination of air drag and thruster firings to meet Shenzhou 9 the following June. After Shenzhou 9 departed, the orbit was raised only to 360 kilometres suggesting the wait this time might be shorter, with the Shenzhou 10 launch occurring just before the year end.

However, a thruster firing 2012 Aug 30 raised the orbit by 11 kilometres and pushed back the potential date into the first few weeks of 2013.

About the time China would have been expected to start reporting readiness of hardware for that particular window, Tiangong 1 raised its orbit again. After a two-impulse manoeuvre on October 15, the orbit became 367 x 374 km, almost matching the highest orbit achieved by Tiangong 1 of 368 x 379 km during 2011 November. It pushed the likely launch date off to 2012 March.

2012 November 10, chinese news media revealed that a date had been set for 2013 June and the mission would last for 15 days. A forward calculation of launch windows undertaken at the time suggested a launch window running from June 8 to June 14 but subject to evolution of the Tiangong orbit over the ensuing months.

2013 Feb 4, Tiangong 1 made a major preparatory manoeuvre prior to the Shenzhou 10 flight. The orbit was raised to prevent re-entry, and set up Tiangong 1 to be back down to Shenzhou operating height early in July. The precedent, set for the Shenzhou 9 mission, is for the orbit to be lowered using thrusters once the launch date is firmly set. That way, a short delay for the mission will not require an immediate re-adjustment of Tiangong 1's orbit.

2013 May 22, Tiangong 1's orbit was lowered to set up the ground track for the Shenzhou 10 mission. The track chosen was 4-5 degrees to the east of the one used for previous missions, probably because of the lower chase orbit needed for the anticipated R-bar approach to docking.

Launch Windows

Tiangong's rendezvous launch windows are explained here and the sets for 2013 June-August are shown on the plot.

The 'Terminal Date' is when Tiangong 1's orbit was expected to have decayed to Shenzhou operating height. For any particular date, it is an estimate based on current orbital altitude and a prediction of the future decay rate. This. in turn is derived from the air drag experienced during the last two complete rotations of the Sun as seen from Earth (54 days).

The plots run until 2012 May 22 when Tiangong adjusted its orbit to set up the ground track for the Shenzhou 10 mission.

On the right hand plot is the level of solar radiation at 10.7 centimetres wavelength reaching the Earth. It is provided by NOAA through its web site. Although there is not a continuous connection between the shapes of the two curves, noticeable peaks and troughs in the weather plot are usually echoed 3-4 days later by the Tiangong forecast date. To best show this, the two date axes have been shifted relatively.

2012 from Jul 10 through to September 4 it was masked by Tiangong's thruster firings. Thruster firing seems to have been used again 2013 January 8 - 21 to counteract a period of very high solar output.

Drag and the ensuing decay rate are also influenced by the cross-sectional area that Tiangong 1 presents to its direction of travel. On at least two occasions prior to Shenzhou 9, Tiangong's orientation was changed to counteract short-term variations in upper air density. The same technique may have been involved during the period of high solar activity January 8 - 21.

NOTE - the Solar Flux plot was re-drawn 2013 Jan 23. It was previously based on space weather data published by the Celestrak website. The Celestrak data is subject to significant changes being made several months after original publication without explanation, making it unreliable as a day-to-day source. Replacing it with actual daily measurements provided by NOAA eliminated Celestrak's errors and it illustrates the connection between orbit decay and solar output much more obviously than before.